title: Postocclusive reactive hyperemia occurs in the rat retinal circulation but not in the choroid.
abstract: OBJECTIVE
  We tested the hypothesis that retinal blood flow has a postocclusive reactive hyperemia response modulated by occlusion duration and metabolic activity, and that choroidal blood flow does not.
  
  
  METHODS
  Anesthetized and paralyzed rats (n = 34) were studied. Retinal and choroidal blood flow was measured by laser speckle imaging and laser Doppler flowmetry, respectively. Blood oxygenation level-dependent functional magnetic resonance imaging (BOLD fMRI) was used to measure changes in relative blood oxygenation of the retinal and choroidal circulations. Transient carotid occlusion was elicited with a hydraulic occluder on the common carotid artery. Several occlusion durations were tested during dark, constant light, and flicker light conditions to modulate metabolic demand. The hyperemia response magnitude was quantified by integrating the area above the blood flow baseline for the 3 minutes after release of the occlusion.
  
  
  RESULTS
  Systemic arterial pressure (108.2 ± 1.4 mm Hg) was unaffected by the carotid occlusions, and was similar among animals and conditions. Retinal blood flow had a reactive hyperemia, but choroidal blood flow did not (e.g., 14 ± 2%.sec versus 0.5 ± 4%. sec after 60-second occlusion). The hyperemia magnitude increased as a nonlinear function of occlusion duration and reached a plateau at occlusion durations < 60 second. The hyperemia magnitude was not altered by different lighting conditions at occlusion durations of 15 and 60 seconds. BOLD fMRI results were similar to the laser-based blood flow measurements.
  
  
  CONCLUSIONS
  The results indicate that metabolic local control has a negligible role in choroidal blood flow regulation and only partially accounts for the blood flow behavior in the retinal circulation.
authors.affiliation: Research Imaging Institute, University of Texas Health Science Center, San Antonio, Texas 78229, USA.
authors.forename: Guang
authors.initials: G
authors.lastname: Li
authors.forename: Jeffrey W
authors.initials: JW
authors.lastname: Kiel
authors.forename: Damon P
authors.initials: DP
authors.lastname: Cardenas
authors.forename: Bryan H
authors.initials: BH
authors.lastname: De La Garza
authors.forename: Timothy Q
authors.initials: TQ
authors.lastname: Duong
mesh_terms: D000818:Animals; D002339:Carotid Arteries / Q000502:physiology; D002829:Choroid / Q000098:blood supply*; D006940:Hyperemia / Q000503:physiopathology*; D017078:Laser-Doppler Flowmetry; D008279:Magnetic Resonance Imaging; D023421:Models, Animal; D051381:Rats; D020318:Rats, Long-Evans; D012039:Regional Blood Flow / Q000502:physiology; D012160:Retina / Q000502:physiology*; D015356:Retinal Artery Occlusion / Q000503:physiopathology*
keywords: BOLD; MRI; choroidal blood flow; laser Doppler flowmetry; laser speckle imaging; reactive hyperemia; retinal blood flow